To the Bottommost Depths

Descend until you reach B29, where you shall find the very last Save Point and Interdimensional Elevator pair. Use them as needed. Following that, continue heading down until you've entered the Bottommost Depths.

Directly ahead, you will find a nameless cocoon. Assault it with your strongest attacks. A ways through the encounter, it will begin casting Drain and Osmose upon your party members, but nothing else. Once the protection crumbles, the screen will turn white, and a strange creature shall reveal itself.

Once it's introduced as the Creator, you will enter another battle. Like the one before it, this fight won't last very long, but this time, your target will fight back. Since it has an Auto-Reflect function, use a Spider Silk to slow it down. Afterward, use Haste, Shell, and Reflect on your own party members. To overcome the Creator's own protection, cast spells like Holy and Flare on your teammates, and they'll hit it for full damage. Furthermore, whenever it copies your spellcasting, the magic will simply reflect back onto it.

Eventually, the second form will collapse, taking the Creator's fractured sanity with it.